1. Field of the Invention
The present invention relates to a communication apparatus, a communication method, and a computer program whereby bidirectional communication is conducted among a plurality of wireless stations like those of a wireless LAN (Local Area Network) or PAN (Personal Area Network). More particularly, the present invention relates to a communication apparatus, a communication method, and a computer program able to belong to a plurality of networks operating on different frequency channels.
In further detail, the present invention relates to a communication apparatus, communication method, and computer program that fulfills multiple roles by simultaneously participating in two or more wireless network systems whose logical network topologies and operating frequency channels differ from one another. More particularly, the present invention relates to a dual role communication apparatus, communication method, and computer program that functions as a station operating in an autonomous distributed manner as part of an ad hoc network or mesh network, while also functioning as a client that communicates with an access point as part of a network in infrastructure mode.
2. Description of the Related Art
Wireless networks have been the subject of much attention, being systems free from the physical wiring involved in the wired communication techniques of the past. Such wireless networks typically implement a wireless LAN standard such as IEEE 802.11a (Institute of Electrical and Electronics Engineers 802.11a), IEEE 802.11b, or IEEE 802.11g. Wireless LANs make flexible Internet connection possible, being able to not only replace existing wired LANs, but also to provide a means to connect to the Internet in public places, such as hotel or airport lounges, train stations, and cafes. Wireless LANs have already been broadly adopted, and it is becoming typical for wireless LAN functions to be built into not only IT equipment such as personal computers (PCs), but also CE (consumer electronics) equipment such as digital cameras and music players.
A typical method used to configure a LAN using wireless technology involves installing a single device, referred to as an access point (AP) or coordinator, that acts as the control station for a given area. The network is then formed under unified control by the control station. The control station arbitrates access timings for a plurality of clients within the network, conducting synchronous wireless communication such that respective clients are synchronized with respect to each other.
Another method of configuring a wireless network known as ad hoc communication has also been proposed, wherein all stations operate peer-to-peer in an autonomous distributed manner, and wherein the stations themselves determine access timings. In particular, ad hoc communication is considered to be suited to small-scale networks made up of a relatively small number of stations positioned in close proximity to one another, whereby arbitrary stations are able to conduct direct, asynchronous wireless communication with each other without using a specific control station.
For example, networking in IEEE 802.11 is based on the concept of the BSS (Basic Service Set). BSS exists in two types: BSS, defined to be in “infrastructure mode” wherein a control station is present; and IBSS (Independent BSS), defined to be in “ad hoc mode” and made up of a plurality of MTs (Mobile Terminals, i.e., stations) with no control station.
Furthermore, in addition to the ad hoc networks defined in the IEEE 802.11 specification, development is also proceeding with respect to communication systems wherein respective stations operating in an autonomous distributed manner connect to each other in a peer-to-peer configuration. For example, “multi-hop communication”, wherein a plurality of stations relay frames, solves the problem of not all peers being contained within the range that can be reached by a single signal. Thus, a large number of stations are able to connect to each other as a result of a plurality of stations relaying frames to each other in multi-hop communication. Currently, one of the IEEE 802.11 task groups (TGs) is proceeding with work to standardize multi-hop communication. In the present specification, a wireless network that conducts multi-hop communication is referred to as a “mesh network”, and the respective stations that constitute the mesh network are referred to as “mesh points” (MPs).
First, network operation in infrastructure mode according to IEEE 802.11 will be described.
In infrastructure mode, the area surrounding an AP that is within the signal range of the AP collectively forms a BSS, thereby forming what is referred to as a cell in a cellular system. Clients (i.e., MTs) present in the proximity of the AP then communicate with the AP and join the network as members of the BSS. More specifically, the AP first transmits a control signal referred to as a beacon at a suitable time interval. MTs capable of receiving the beacon thereby confirm the presence of a nearby AP and subsequently establish a connection with that AP.
FIG. 11 illustrates exemplary operation of an IEEE 802.11 network in infrastructure mode. In the example shown in FIG. 11, the station labeled STA0 is operating as the AP, while the other stations labeled STA1 and STA2 are operating as MTs. As shown in the chart on the right side of FIG. 11, the STA0 acting as the AP transmits a beacon at a fixed time interval. The AP manages the transmission interval of the beacon internally as a parameter referred to as the Target Beacon Transmit Time (TBTT), and activates the beacon transmission procedure every time the current time reaches the TBTT. In addition, the beacon issued by the AP also includes a Beacon Interval field. Using the information from the Beacon Interval field and the time of reception of the beacon itself, nearby MTs are able to determine the next TBTT.
At this point, the BSS may enter a Power Save mode as appropriate, whereby power consumption can be reduced as a result of each MT performing only intermittent operations for receiving signals. In Power Save mode, at least a portion of the MTs in the BSS operate in a Sleep mode, alternating between an Awake state wherein the transceiver is operative, and a Doze state wherein power to the transceiver is cut. Since the MTs are able to determine the transmit time of the next beacon from a received beacon, in Sleep mode an MT may cut power to the receiver and enter a low-power state when not expecting an incoming signal, the low-power state lasting until the next TBTT or until a plurality of TBTT intervals have elapsed. An MT that is not in Sleep mode is referred to as being in Active mode, wherein the transceiver is in continuous operation (see FIG. 12).
The AP centrally manages the timings whereby respective MTs in Sleep mode switch to Awake state, transmitting frames to a particular MT to match the timings whereby that MT enters Awake state. In so doing, the AP supports low-power operation. More specifically, when there exist packets addressed to an MT in Sleep mode, the packets are queued internally rather than immediately transmitted, and information indicating the existence of queued packets is included in the beacon signal to notify the destination MT. The above information that is included in the beacon signal is referred to as the TIM (Traffic Indication Map). An MT in Sleep mode receives and analyzes the beacon signal from the AP, and by referring to the TIM, the MT is able to determine whether or not the AP is buffering traffic addressed to the MT itself. Thus, if an MT in Power Save mode determines that there is traffic to be received, that MT transmits a request signal addressed to the AP that requests the transmission of packets addressed to that MT. Subsequently, the AP transmits the queued data to the MT in response to the request signal.
Network operation in ad hoc mode according to IEEE 802.11 will now be described.
In the ad hoc mode (IBSS) of IEEE 802.11, MTs autonomously define an IBSS upon mutually confirming the presence of a plurality of MTs. The MT group then determines a TBTT at a fixed interval. The beacon transmission interval is announced by means of a parameter within the beacon signal, thereby enabling respective MTs to calculate the next TBTT after having received a beacon signal once. Subsequently, when an individual MT determines that the TBTT has been reached by referring to its internal clock, the MT delays operation for a random amount of time (referred to as a random backoff). If after the random backoff the MT determines that no other MT has yet transmitted a beacon, then the MT transmits a beacon itself. MTs capable of receiving this beacon are then able to join the current IBSS.
FIG. 13 illustrates exemplary operation of an IEEE 802.11 network in ad hoc mode. In the example shown in FIG. 13, the two stations operating as MTs and labeled STA1 and STA2 constitute an IBSS. In this case, either MT belonging to the IBSS may transmit a beacon each time the TBTT is reached. In addition, there also exist cases wherein beacons sent from respective MTs collide.
The IEEE 802.11 specification also describes a Power Save mode for IBSS, whereby MTs are able to cut power to the receiver and enter Doze state as appropriate. A predetermined time period starting from the TBTT is also defined as the ATIM Window (Announcement Traffic Indication Message Window). During the period before the ATIM Window ends, all MTs belonging to the IBSS enter Awake state, and thus even MTs fundamentally operating in Sleep mode are able to receive incoming signals during this time period. In addition, MTs are able to enter Doze state starting from the time the ATIM Window ends and lasting until the next TBTT.
In the case where a particular MT has information to be transmitted to another station, the MT transmits an ATIM packet to the above destination during the ATIM Window. In so doing, the transmitting MT notifies the receiving station that information for transmission is being held. Meanwhile, the MT that receives the ATIM packet activates its receiver and does not enter Doze state until the information from the station that transmitted the ATIM packet has been fully received.
FIG. 14 illustrates exemplary operation in the case where three MTs labeled STA1, STA2, and STA3 are present within an IBSS. When the TBTT is reached, each MT STA1, STA2, and STA3 activates a backoff timer while monitoring the state of the medium for a random amount of time. In the example shown in FIG. 14, the timer in STA1 expires first, and thus STA1 transmits the beacon. Since STA1 has transmitted the beacon, upon receiving the beacon signal STA2 and STA3 refrain from transmitting another beacon signal.
In the present example, STA1 is holding information addressed to STA2, while STA2 is holding information addressed to STA3. In this case, after STA1 has transmitted and STA2 has received the beacon, STA1 and STA2 both respectively activate backoff timers again while monitoring the state of the medium for random amounts of time. In the example shown in FIG. 14, the timer in STA2 expires earlier, and thus STA2 first transmits an ATIM message addressed to STA3. Upon receiving the ATIM message, STA3 briefly waits for an amount of time known as the Short Interframe Space (SIFS), and then replies to STA2 with an ACK (Acknowledge) packet indicating that the ATIM message was received. Once the transmission of the ACK by STA3 is completed, STA1 once again activates a backoff timer while monitoring the state of the medium for a random amount of time, and when the timer expires, STA1 transmits an ATIM packet addressed to STA2. Subsequently, STA2 waits one SIFS and then replies to STA1 with an ACK packet indicating that the ATIM packet was received.
After exchanging ATIM packets and ACK packets as described above, STA3 activates its receiver for the remaining duration of the ATIM Window in order to receive any further information from STA2, while STA2 likewise activates its receiver in order to receive any further information from STA1.
Once the ATIM Window ends, STA1 and STA2, still holding information to be transmitted, subsequently wait for an amount of time referred to as the Distribute Interframe Space (DIFS), an interval corresponding to the minimum amount of time that the medium is idle. After waiting one DIFS, both STA1 and STA2 respectively activate backoff timers while monitoring the state of the medium for random amounts of time. In the example shown in FIG. 14, the timer in STA2 expires earlier, and thus STA2 first transmits a data frame addressed to STA3. Subsequently, after waiting one SIFS, STA3 replies to STA2 with an ACK packet indicating that the data frame was received.
Once transmission of the above data frame is completed, STA1 waits one DIFS, and then once again activates its backoff timer while monitoring the state of the medium for a random amount of time. Once the timer expires, STA1 transmits a data frame addressed to STA2. Subsequently, STA2 waits one SIFS, and then replies to STA1 with an ACK packet indicating that the data frame was received.
In the above sequence, an MT that neither receives an ATIM packet during the ATIM Window nor holds information to be transmitted to another station is able to cut power to its transceiver until the next TBTT, thereby reducing power consumption.
The operation of a mesh network will now be described.
Wireless communication systems have been proposed wherein, for example, respective stations transmit beacons to each other that include network-related information, the information subsequently being used to establish a network. Using such beacons, stations in the wireless communication system are able to conduct high-level decisions with respect to the states of other stations. (See, for example, Japanese Unexamined Patent Application Publication No. 2003-304115). A mesh network can be established using a similar method.
FIG. 15 shows an exemplary communication sequence occurring in a wireless communication system wherein respective stations communicate in an autonomous distributed manner by exchanging beacon signals. In the example shown in FIG. 15, two stations exist within communicable range of each other and herein act as the stations participating in the network. Each station sets a TBTT (Target Beacon Transmission Time) individually, and transmits a beacon signal at regular intervals. In addition, in order to extract information about neighboring MTs, each station also periodically receives the beacon signal from the other station as appropriate.
In addition, it is herein assumed that STA1 enters Sleep mode and cuts power to its transceiver when appropriate, and that an MT in Power Save mode alternates between an Awake state wherein the transceiver is operative, and a Doze state wherein power to the transceiver is cut (as described earlier).
FIG. 16 illustrates an example of how data transmission from STA1 to a station STA0 occurs. The upper part of FIG. 16 shows a packet transfer sequence occurring between STA0 and STA1, while the lower part of FIG. 16 shows the operational state of the STA0 transceiver, STA0 herein acting as the data recipient (in the lower part of FIG. 16, the high level indicates Awake state, and the low level indicates Doze state). Note that when either transceiver is in Doze state, the corresponding station is in Power Save mode. Likewise, when either transceiver is in Awake state, the corresponding station is not in Power Save mode for that period of time.
Once an MT has transmitted a beacon, the MT enters a listen period for a set period of time during which the receiver is operative. If the MT does not receive any traffic addressed to itself during the listen period, then the MT may cut power to the transceiver and switch to Power Save mode. In the example shown in FIG. 16, STA0 transmits the beacon B0-0 and subsequently activates its receiver for a short period. Since STA1 then transmits a packet addressed to STA0 during this period, STA0 is able to receive the packet.
The beacon signal also contains information referred to as the TIM (Traffic Indication Map). The TIM is information notifying whether or not there currently exists addressed to a particular station. By referring to the TIM, a beacon-receiving station is able to determine whether or not that station should receive incoming signals. Each MT periodically receives beacon signals from surrounding MTs and analyzes the TIM therein. If a particular MT confirms that data addressed to itself does not exist, then that MT cuts power to its receiver and enters Sleep mode. If a particular MT confirms that data addressed to itself does exist, then that MT transitions to a data-receiving state without entering Sleep mode.
FIG. 16 illustrates an example wherein the TIM in the beacon B1-1 indicates that STA1 is calling STA0. Upon receiving the beacon, STA0 issues a response to the call (0). Upon receiving the response, STA1 subsequently confirms that STA0 is in a receiving state, and then transmits a packet addressed to STA0 (1). Upon receiving the packet, STA0 confirms that the packet was received normally, and then transmits an ACK (2).
The sequence of received packets with respect to STA0 as indicated by (0), (1), and (2) in FIG. 16 will now be described in further detail.
When a station operating in Power Save mode learns that another station is holding data addressed to itself, the station transmits a poll frame (corresponding to frame (0) in FIG. 16) that triggers transmission by the other station. Two types of poll frames exist: a poll frame that triggers transmission of just one packet (hereinafter referred to as type # A), and a poll frame that allows the sender to transmit a plurality of packets (hereinafter referred to as type # B).
The left side of FIG. 17 illustrates a sequence of received packets wherein a type # A poll frame is used.
Upon receiving a type # A poll frame (0) from STA-0, STA-1 transmits a single data packet (1) in response. The header of the above data packet contains a flag indicating whether or not there exist subsequent packets to be transmitted. Thus, by receiving the data packet from STA-1, STA-0 is able to ascertain whether or not STA-1 wants to subsequently transmit additional packets. In FIG. 17, Data-ct corresponds to the data packet that notifies STA-0 that STA-1 wants to transmit additional packets, while Data-fin corresponds to a data packet indicating that there are no subsequent packets to be transmitted.
In the example shown in FIG. 17, the data packet first transmitted by STA-1 (i.e., Data-ct) notifies STA-0 that there exist subsequent packets. For this reason, after receiving Data-ct normally, STA-0 transmits the type # A poll frame (0) again in order to receive the additional packets. STA-0 then attempts to receive the subsequent frames.
The data packet transmitted by STA-1 in response to the second type # A poll frame is Data-fin, and thus contains information indicating that STA-1 does not wish to transmit any further packets. For this reason, upon receiving Data-fin normally, STA-0 replies with an ACK, terminates the process for receiving the series of data from STA-1, and then transitions back to a state wherein Sleep mode is possible.
The right side of FIG. 17 illustrates a sequence of received packets wherein a type # B poll frame is used.
The transmission of the type # B poll frame (0′) by STA-0 implies that STA-0 promises to keep its receiver on until Data-fin is received from STA-1. Upon receiving the type # B poll frame from STA-0, STA-1 replies with an ACK (1′) indicating that the above message was correctly received. After this point, STA-1 may transmit any number of Data-ct packets before transmitting Data-fin. The transmission of data to STA-0 is terminated as a result of STA-1 transmitting Data-fin.
By receiving Data-fin from STA-1, STA-0 confirms that STA-1 does not wish to transmit any further packets. Subsequently, STA-0 replies with an ACK, terminates the process for receiving the series of data from STA-1, and then transitions back to a state wherein Sleep mode is possible.
It should be appreciated that a communication sequence similar to that shown in FIG. 17 can be applied to a sequence in infrastructure mode, wherein an MT in Power Save mode receives data from an access point. More specifically, by transmitting a type # A poll frame or type # B poll frame, an MT is able to pull data from the access point.
As described in the foregoing, several different logical network topologies are possible in a wireless LAN system. In addition, there also exist dual role devices, whereby a single physical station is able to logically join a plurality of networks simultaneously.
FIGS. 18 and 19 illustrate how, in a wireless LAN system made up of a plurality of different logical networks, a single physical station fulfills dual roles with respect to two logical networks.
In the example shown in FIG. 18, STA-A constitutes a logical network A, while STA-D constitutes a logical network D. In this communication environment, the single physical station STA-C participates in the network A via the link A-C, while also simultaneously participating in the network D via the link D-C. In this example, it is possible for both the network A and the network D to operate in infrastructure mode, and STA-A and STA-d may both be operating as access points.
In contrast, in the example shown in FIG. 19, the network A may also be operating in ad hoc mode or as an autonomous distributed network such as a mesh network. In this case, the dual role STA-C is able to communicate with the access point STA-D as an MT in the network D, while also communicating directly with the respective stations STA-A and STA-B as either an MT or MP in the other network A.
Consider now the merits of a single physical station fulfilling dual roles by taking as an example the case wherein such a device simultaneously participates in an autonomous distributed network such as an ad hoc network or mesh network, while also participating in a network operating in infrastructure mode.
In. FIG. 19, STA-C is in a state of bidirectional communication with STA-A and STA-B in an autonomous distributed mode, wherein the three stations are herein assumed to be running an application such as a competitive multiplayer game. The present example assumes the case wherein STA-C is controlling the game on the basis of information downloaded from the Internet, while simultaneously proceeding with the competitive multiplayer game with STA-A and STA-B. In this case, STA-C is able to communicate with the access point STA-D in infrastructure mode and thereby download information from the Internet via STA-D, while simultaneously maintaining communication with STA-A and STA-B in an autonomous distributed mode. If a station is able fulfill dual roles and thereby simultaneously participate in a plurality of logical networks as shown by way of example in FIG. 19, then it is anticipated that a great variety of network services can be provided.
Furthermore, although the above plurality of logical networks may be established on the same channel, the networks do not interfere with each other if operated on different channels, and thus it is possible to provide more communication bandwidth. Consequently, there is demand for a dual role device able to belong to a plurality of networks operating on different frequency channels.
However, if a station that fulfills dual roles by belonging to a plurality of networks has only one modem, then that station only transmits and receives on one frequency channel at any given time. For this reason, such a station may be unable to simultaneously participate in different logical networks operating on different frequency channels.
For example, given the wireless LAN system configuration shown in FIG. 18, if the network A and the network D are operating on different frequency channels, and additionally, if the station STA-C belonging to both networks has only one modem, then the link A-C and the link D-C are not established simultaneously. However, in order to improve the connectivity of dual role devices, there is demand for the ability for such a device to belong to a plurality of networks operating on different channels with the use of only one modem.